According to US20030061805A1 (JP-A-2003-83048) and US20040011027A1 (JP-A-2004-11585), a secondary air supply system includes an air pump that supplies secondary air to the upstream of a catalyst that purifies exhaust gas in an exhaust passage of the engine. This operation enhances purification (oxidization) of HC and CO contained in exhaust gas, and accelerates warming-up of the catalyst with heat generated by reaction (oxidization) of HC and CO.
When a malfunction occurs in the secondary air supply system, exhaust emission may be degraded. According to the secondary air supply system disclosed in US20030061805A1 and US20040011027A1, a pressure sensor is provided to evaluate abnormality of the secondary air supply system in accordance with pressure of secondary air detected using the pressure sensor in an abnormality diagnostic operation.
Variation in discharge capacity of the air pump, which supplies secondary air, is caused by variation in voltage of a power source (battery). Specifically, when voltage (power source voltage) of the power source decreases, the discharge capacity of the air pump becomes low. In this situation, it is conceivable that the abnormality diagnostic operation is prohibited not to make an erroneous determination of abnormality in the secondary air supply system.
However, in general, secondary air is supplied in a catalyst warm-up period immediately after the engine is started. When the abnormality diagnostic operation is prohibited when the power source voltage is low, the abnormality diagnostic operation cannot be performed in a normal operation, in which secondary air is normally supplied, due to undervoltage of the power source. As a result, frequency of the abnormality diagnostic operation becomes low. In particular, the air pump is supplied with a large current. Therefore, when the battery is substantially exhausted, the air pump is always in an undervoltage condition, and consequently the abnormality diagnostic operation cannot be performed at all.
A particular charge system performs a charge control when an engine is started, or the like. Specifically, the power source voltage is gradually returned to an initial voltage when an undervoltage condition is detected in the charge control, so that drivability is restricted from being degraded due to quick change in the power source voltage. In this case, the undervoltage condition continues for a long period due to the charge control, and consequently the abnormality diagnostic operation is prohibited for a long period. As a result, frequency of the abnormality diagnostic operation becomes low.
Furthermore, the discharge capacity of the air pump changes corresponding to the power source voltage. Accordingly, pressure of secondary air detected using the pressure sensor changes corresponding the power source voltage. Therefore, when the abnormality diagnostic operation of the secondary air supply system is performed in accordance with the detection signal of the pressure sensor, an erroneous determination may be made due to variation in detection signal of the pressure sensor caused by variation in power source voltage.